1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, it relates to a fringe field switching mode liquid crystal display device having high transmittance and high aperture ratio.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are being developed as the next generation display devices because of their characteristics of light weight, thin profile, and low power consumption. In general, an LCD device is a non-emissive display device that displays images by making use of a refractive index difference through utilizing optical anisotropy properties of a liquid crystal material interposed between an array substrate and a color filter substrate. Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
A conventional LCD device, generally, uses twisted nematic (TN) mode liquid crystal, the orientation of which is parallel to substrates and is continuously twisted from one substrate to another substrate by 90 degrees. However, the TN mode LCD device has disadvantages of a narrow viewing angle and slow response characteristics.
To solve the above problems, various modes, such as a multi-domain TN structure and an optically compensated birefringence (OCB) mode, have been proposed. In the multi-domain TN structure, a pixel is divided into several domains. The process of manufacturing the multi-domain is complicated, and there exists limitation in improving the viewing angle. The OCB mode has wide viewing angles and fast response time. However, in the OCB mode, it is difficult to control and maintain the liquid crystal material stably due to bias voltage.
In-plane switching mode liquid crystal display (IPS-LCD) devices, recently, have been proposed as a new display mode. In the IPS-LCD devices, electrodes for driving liquid crystal molecules are formed on the same substrate.
FIG. 1 is a cross-sectional view illustrating the concept of a related art IPS-LCD device. As shown in FIG. 1, an upper substrate 10 and a lower substrate 20 are spaced apart from each other, and a liquid crystal layer 30 is interposed therebetween. The upper substrate 10 and lower substrate 20 are often referred to as a color filter substrate and an array substrate, respectively. A common electrode 22 and a pixel electrode 24 are positioned on the lower substrate 20. The common electrode 22 and pixel electrode 24 are positioned such that they are parallel to each other. On a surface of the upper substrate 10, a color filter layer (not shown) is commonly positioned to correspond to an area between the pixel electrode 24 and the common electrode 22 of the lower substrate 20.
A voltage applied across the common electrode 22 and pixel electrode 24 produces an in-plane electric field 26 through liquid crystal molecules of the liquid crystal layer 30. The liquid crystal molecules have a positive dielectric anisotropy, and thus the liquid crystal molecules will align so as to be in parallel with the electric field 26. The viewing angles can range 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD device, for example.
FIG. 2 is a plane view of an array substrate according to the related art IPS-LCD device. As shown in FIG. 2, a gate line 40 and a data line 42 cross each other to define a pixel region P. At a crossing of the gate line 40 and the data line 42, a thin film transistor T is formed. A common line 44 is spaced apart from the gate line 40, and in the pixel region P, a plurality of common electrodes 46 extends from the common line 40 parallel to the data line 42.
A first pixel connecting line 48 is connected to the thin film transistor T, and a plurality of pixel electrodes 50 extends from the first pixel connecting line 48 alternating with the plurality of common electrodes 46. A second pixel connecting line 52 connects ends of the plurality of pixel electrodes 50, and the second pixel connecting line 52 overlaps the common line 44. The overlapped common line 44 and second pixel connecting line 52 form a storage capacitor CST with an insulating layer interposed therebetween.
Spaces between the common electrodes 46 and the pixel electrodes 50 correspond to aperture areas A, where liquid crystal molecules are driven by an electric field parallel to a substrate. In the above array substrate, there exist 4 blocks of 4 aperture areas A in one pixel. That is, in the pixel region P, three common electrodes 46 and two pixel electrodes 50 alternate with each other.
The common electrodes 46 include two first common electrodes 46a that are near by the data line 42 and a second common electrode 46b that is disposed between the first common electrodes 46a. To minimize cross-talk between the data line 42 and the pixel electrodes 50 and to prevent light leakage, the first common electrodes 46a should have a wider width than the second common electrode 46b, and this reduces an aperture ratio.
To improve the aperture ratio and transmittance of the related art IPS-LCD device, fringe field switching (FFS) mode LCD device has been suggested. The FFS mode LCD device has a square common electrode of an island shape corresponding to the pixel region and a pixel electrode consisting of a plurality of rods that are spaced apart from each other and forming slits. The common electrode and the pixel electrode overlap each other with an insulating layer interposed therebetween. In the FFS mode LCD device, since electric fields are induced every several angstroms, the electric fields are strong, and thus even the liquid crystal molecules over the electrodes can be arranged by the electric fields. In addition, as the common electrode and the pixel electrode are formed of a transparent conductive material, the aperture ratio may be improved.
FIG. 3A is a plan view illustrating an FFS mode LCD device according to the related art, and FIG. 3B is a cross-sectional view along the line IIIB-IIIB of FIG. 3A. FIG. 3A shows mainly an array substrate of the FFS mode LCD device, and FIG. 3B illustrates a cross-section of the FFS mode LCD device including a liquid crystal layer in a corresponding cutting area.
In FIG. 3A, a gate line 62 and a data line 78 cross each other to define a pixel region P. A thin film transistor T is formed at a crossing of the gate line 62 and the data line 78. In the pixel region P, a plurality of pixel electrodes 82 connected to the thin film transistor T are spaced apart from each other. A common electrode 68 extends below the plurality of pixel electrodes 82.
More particularly, the thin film transistor T includes a gate electrode 64, a semiconductor layer 72, a source electrode 74 and a drain electrode 76. A first pixel connecting line 84 is connected to the drain electrode 76, and the plurality of pixel electrodes 82 extends from the first pixel connecting line 84. Ends of the plurality of pixel electrodes 82 are electrically connected to each other by a second pixel connecting line 86. The common electrode 86 of each pixel region P is connected to a common line 66, which is spaced apart from and is parallel to the gate line 62.
The common electrode 68 and the plurality of pixel electrodes 82 are formed of a transparent conductive material through different processes. The common line 66 is formed of the same material through the same process as the gate line 62. The common electrode 68 directly contacts the common line 66 without an insulating layer therebetween, and the plurality of pixel electrodes 82 are disposed over the common electrode 68 with an interposed insulating layer (not shown).
Operations of the FFS mode LCD device will be explained hereinafter through the cross-sectional structure of FIG. 3B. In FIG. 3B, a square common electrode 68 is formed on a first substrate 60, and a first insulating layer 70 covers the common electrode 68. A plurality of pixel electrodes 82 are formed on the first insulating layer 70 over the common electrode 68. The plurality of pixel electrodes 82 form slit shapes that are spaced apart from each other. A first alignment layer 88 covers the plurality of pixel electrodes 82.
A second substrate 90 is spaced apart from and faces the first substrate 60. A color filter layer 92 and a second alignment layer 94 are sequentially formed on an inner surface of the second substrate 90. A liquid crystal layer 96 is interposed between the first and second alignment layers 88 and 94.
In the FFS mode, the liquid crystal molecules between the electrodes are rotated by a lateral electric field to be parallel with the substrates, and then the liquid crystal molecules over the electrodes rotate due to vertical and lateral electric fields around the electrodes and an elastic force of the liquid crystal. That is, as light is also transmitted over the electrode, the transmittance is high. Moreover, rotation rates of the liquid crystal molecules are different in one pixel, and thus color shift is decreased by self-compensating effects.
If the electrodes may have stripe shapes, to form a lateral electric field, an alignment direction makes an angle of about 60 degrees with the gate line, which is a base line of 0 degree. Since the alignment direction is also inclined with respect to light transmission axes of polarizers (not shown), ranges of the viewing angle become non-uniform. The alignment direction may be within a range of 90 degrees to -270 degrees with respect to the base line, and the light transmission axes of the polarizers, which are 0 degrees and 90 degrees with respect to the base line, respectively, cross each other at a right angles. Therefore, viewing angle characteristics are lowered at directions of 45 degrees and 135 degrees.
In addition, there exists color shift difference according to angles of all directions, and thus the viewing angle characteristics are decreased.